My house is 15 years old. I have never done any air conditioner maintenance since the house was new. My compressor is a Lennox HS22-261U-2P. When I turn the thermostat to "cool", the compressor buzzes loudly but the motor never turns. The fan can be easily turned by hand. What is the likely problem and is there an easy way to diagnose? I am handy with mechanical assembly and with a voltmeter, so I am hoping that I can do this repair myself.

Check your supply voltage to outdoor unit. If adequate the problem may be a failed capacitor.

I opened the side of the compressor. The buzz is coming from the relay (contactor?) that I assume is used to connect the 220v power to the compressor motor. When you say to check the source voltage, I am assuming that you mean the 220v AC. Can you confirm this?

Are there any special precautions that I need to take when replacing the capacitor? I'm assuming that the capacitor may hold a high voltage for a long period unless there is something that is bleeding off the voltage once the power is disconnected. I will make sure that the power is completely disconnected before doing anything. But do I need to wait a while or manually discharge the capacitor somehow before trying to remove the leads from it? Please let me know if there are any special safety procedures that I should follow when making this repair. BTW, would you expect the relay to be damaged as well and need replacement? My guess is that it may have buzzed for many hours before I initially realized it wasn't working.

I took a closer look at the contactor. The terminals are fairly corroded as can be seen in the attachment. I could clean them off and reattach the lugs if this is likely to help. What would you recommend?

You could shut power off and clean, however a contactor does not cost that much. My advise would be to replace

Your contactor is probably stuck not giving you power to your compressor or fan in the middle of your contactor there is a button see if you push it in with a screw driver if anything comes on if it does not push in easily your contactor is bad. If it is already pulled in then look at your capacitor.

That corrosion is on the solenoid coil terminals and is probably dropping the voltage to the coil and causing it chatter and not pull in, I agree with Esquire1, replace contactor first and then have capacitor checked out. Good luck, Mike

I replaced the contactor, and now the compressor seems to be working correctly. Many thanks to everyone who contributed answers to my questions!